Fire detectors have been widely installed in both commercial buildings and residential structures to protect their inhabitants and other contents. These fire detectors are generally of the following three types: flame detector, thermal detector, or smoke detector. These three classes of detectors correspond to the three primary properties of a fire: flame, heat, and smoke.
Flame Detectors: A flame detector responds to the optical energy radiated from a fire and typically responds to nonvisible wavelengths. One class of these detectors operates in the ultraviolet (UV) region below 4,000.ANG., and a second class of these detectors operates in the infrared region above 7,000.ANG.. To prevent false alarms from other sources of UV or infrared light, flame detectors are constructed to respond only to radiation in one of these two regions that varies in intensity at a frequency characteristic of typical flicker frequencies of flames (i.e., a frequency in the range of 5 to 30 Hertz).
Although they exhibit a low rate of false alarms, flame detectors are relatively complex and expensive. Thus, these detectors are generally used only for applications in which cost is not a significant factor. For example, this type of detector is commonly used in industrial environments such as aircraft hangers and nuclear reactor control rooms.
Heat (Thermal) Detectors: Heat from a fire is dissipated by both laminar convective and turbulent convective flow. The convective flow is produced by the rising hot air and combustion gases within the plume of the fire. The two basic types of thermal detectors are threshold temperature detectors, which detect when a threshold temperature has been exceeded, and rate of rise detectors, which detect when a threshold rate of temperature increase has been exceeded.
Threshold temperature detectors are reliable, stable and easy to maintain, but are relatively insensitive. This type of detector is rarely used, especially in buildings having high airflow ventilation and air conditioning systems. Rate of rise detectors are typically used only in environments in which fires are expected to be fast-burning, such as chemical fires. The threshold for these detectors is typically about 15 degrees Fahrenheit per minute. Unfortunately, there is a significant rate of false detections for both types of thermal detectors.
A third type of thermal detectors has been recently introduced that indicates the presence of a fire only if both the temperature and rate of rise of the temperature exceed their respective thresholds. Although this eliminates a high fraction of the false detections, it also makes these detectors highly susceptible to failing to detect the actual occurrence of a fire. This requires that the location of these detectors be carefully selected. As a result, this type of fire detector is seldom used in residences. This type of detector is typically used in the same type of environment as the rate of rise detector.
Smoke Detectors: Since 1975, the United States has experienced remarkable growth in the use of home smoke detectors, principally single-station, battery-operated, ionization-mode smoke detectors. This rapid growth, coupled with clear evidence from actual fires and fire statistics of the lifesaving effectiveness of detectors, has made the home smoke detector the fire safety success story of the past two decades.
In recent years, however, studies of the operational status of smoke detectors in homes has revealed the alarming statistic that as many as one-fourth to one-third of all smoke detectors are nonoperational at any given time. Over half of the nonoperational smoke detectors are missing batteries. The rest have dead batteries or are broken. Homeowners' frustration over nuisance alarms (also referred to as "false alarms") is the principal reason for the missing batteries. Nuisance alarms are detector activations caused not by uncontrolled fires but by controlled fires, such as cooking flames. These nuisance alarms are also caused by nonfire sources, such as moisture vapor from someone taking a shower, dust or debris stirred up during the cleaning of living quarters, or oil vapors from cooking. To understand why the false alarm rate for currently available fire detectors is undesirably high, one must understand the standards that have been set for the performance of fire detectors and fire detection systems.
The present standard for common household fire detectors in the United States is contained in UL 217 Standard for Single and Multiple Station Smoke Detectors (Third Edition), which has been approved by the American National Standard Institute and is hereinafter referred to as ANSI/UL 217. ANSI/UL 217 covers (1) electrically operated single and multiple station smoke detectors intended for open area protection in ordinary indoor locations of residential units in accordance with the Standard for Household Fire Warning Equipment, NFPA 74, (2) smoke detectors intended for use in recreational vehicles in accordance with the Standard for Recreational Vehicles, NFPA 501C, and (3) portable smoke detectors used as "travel" alarms. ANSI/UL 268 is a similar standard for larger fire alarm systems that are typically installed in office buildings and commercial structures.
Recognizing that different types of fires have different characteristics, ANSI/UL 217 contains tests for paper, wood, gasoline, and polystyrene fires. The procedure for performing tests characteristic of each of these fires is set forth in paragraph 42 of ANSI/UL 217. According to paragraph 42.1 of ANSI/UL 217, the maximum response time for an approved fire detector is four minutes for paper and wood fire tests, three minutes for a gasoline fire test, and two minutes for a polystyrene fire test. Because the highest maximum response time is four minutes, it is common to refer to a maximum response time of four minutes for a residential fire detector without reference to the paper or wood fire tests. Although ionization flame detectors sold for residential use could be set to have a maximum response time of fewer than four minutes, most residential detectors have a maximum response time of about four minutes to minimize the occurrence of false alarms while still meeting the mandated response time standard.
Ionization-mode smoke detectors are prone to nuisance alarms because they are more sensitive to invisible particulate matter than to visible particulate matter. Because the alarm threshold must be set low enough for an alarm to be declared when primarily visible particulate matter is present, and because by that point considerable invisible particulate matter has been generated, false alarms often occur. Ionization type smoke detectors are prone to false alarms because they are more sensitive to invisible particulate matter (from 0.01 to 2 micron in largest dimension) than to visible particulate matter (from 2 microns to 5 microns in largest dimension). The detection threshold must be set quite low so that ionization type detectors can quickly detect those fires that do not produce a great deal of invisible particulate matter. This causes ionization type smoke detectors to issue false alarms when they encounter small amounts of invisible particulate matter produced by nonfire sources.
The problem of frequent false alarms among ionization smoke detectors, which results in a significant portion of them at any given time being unreliable, has led to the increased use in recent years of another type of smoke detector, the photoelectric smoke detector. Photoelectric smoke detectors work best for visible particulate matter and are relatively insensitive to invisible particulate matter. They are therefore less prone to nuisance alarms. However, their drawback is that they do not respond well to smoldering fires in which the early particulate matter generated is mostly invisible. To overcome this drawback, the fire alarm threshold of photoelectric smoke detectors must be set very low to meet the ANSI/UL 217 or ANSI/UL 268 certification requirements. Setting the fire alarm threshold for photoelectric smoke detectors so low leads to frequent false alarms. Thus the problem of nuisance false alarms for smoke detectors seems unavoidable.
Over the years the problem has been recognized but has not been solved. Frequent false alarms are not just a harmless nuisance; they may lead people to disarm smoke detectors by removing the battery to prevent such annoyances. This can be dangerous, especially when such people forget to re-arm their smoke detectors by replacing the battery. Frequent false alarms in fire detection systems in large buildings pose a safety hazard by leading occupants and fire fighters and other safety personnel to believe that any alarm is likely to be false. Regardless of the degree to which safety is stressed, the typical human reaction is to respond with less urgency to an alarm if frequent false alarms have been encountered in the past.
Another aspect of present-day smoke detectors that is often discussed but seldom addressed is the slowness of these detectors in detecting fire. The current ANSI/UL 217 and ANSI/UL 268 fire detector certification codes were developed years ago according to the then available fire detection technology--the smoke detector. Over the past two decades, workers in the fire fighting and prevention industries have been critical of the speed of response of the smoke detector. Obviously, increasing smoke the sensitivity of detectors by lowering their light obscuration detection thresholds speeds up their response. However, it also increases the nuisance alarm rates. It is clear that a better fire detector is needed.
Photoelectric smoke detectors can be divided into projected beam detectors and reflected beam detectors. The projected beam detector generally contains a series of pipes connected to the photoelectric detector. Air is drawn into the piping system by an electric exhaust pump. The photoelectric detector is usually enclosed in a metal tube with the light source mounted at one end and the photoelectric cell at the other end. Typically for this type of detector to be effective it must be long enough to accommodate a light beam of at least one meter in length so that small amounts of smoke will produce measurable amounts of attenuation. Unfortunately, this makes these detectors inconvenient to install. When visible smoke is drawn into the tube, the intensity of the light beam received by the photoelectric cell is reduced by the smoke particles. This reduction in intensity is detected by an electrical circuit connected to the photoelectric cell, which, in turn, activates the alarm. The projected beam or smoke obscuration detector was one of the first types of smoke detectors to be developed. In addition to its use on ships, this detector is commonly used to protect high-value compartments of storage areas and to provide smoke detection for plenum areas and air ducts.
The reflected light beam smoke detector has a light beam of only 5-7 cm in length, making it suitable for housing in the round, white, approximately 15 cm diameter cases, which will be familiar to most people. A reflected beam visible light smoke detector contains a light source, a photoelectric cell mounted at a right angle to the light source, and a light catcher mounted opposite to the light source.
For the past two decades, ionization smoke detectors have dominated the fire detector market. One of the reasons for this is that the other two classes of fire detectors, the flame and thermal detectors, are appreciably more complex and costly than ionization detectors. Therefore, flame and thermal detectors are primarily used in specialized high-value and unique-protection areas. In recent years, because of their relatively high cost, the photoelectric smoke detectors have significantly fallen behind in sales to the ionization types. Ionization detectors are generally less expensive and easier to use and can usually operate for a full year with one 9-volt battery. Today, over 90 percent of residences that are equipped with fire detectors use ionization smoke detectors.
Despite their low cost, relatively maintenance-free operation, and wide acceptance by consumers, these smoke detectors are not without problems and are certainly far from ideal. A number of significant drawbacks for ionization prevent them from operating as successfully as other early warning fire detectors.
One drawback to smoke detectors is the relatively slow and unpredictable dispersal characteristics of smoke. Unlike ordinary gases, smoke is a complex, sooty molecular cluster that consists mostly of carbon. It is much heavier than air and thus diffuses much more slowly than the gases we encounter every day. Therefore, if the detector happens to be some distance from the location of the fire, significant time will elapse before enough smoke gets into the sampling chamber of the smoke detector to trigger the alarm. Another drawback is the considerable variation in the amount of smoke produced by a fire. This depends on the composition of the material that catches fire. For example, oxygenated fuels such as ethyl alcohol and acetone generate less smoke than the hydrocarbons from which they are derived. Thus, under free-burning conditions, oxygenated fuels such as wood and polymethylmethacrylate generate substantially less smoke than hydrocarbon polymers such as polyethylene and polystyrene. Indeed, a small number of pure fuels, such as carbon monoxide, formaldehyde, metaldehyde, formic acid, and methyl alcohol, burn with nonluminous flames and do not produce smoke at all.
In an attempt to address the deficiencies, efforts have been made to develop a new type of fire detector. In this regard, it has been known for a long time that as a process, fire can take many forms, all of which involve a chemical reaction between combustible species and oxygen from the air. In other words, fire initiation is an oxidation process because it invariably entails the consumption of oxygen at the beginning. The most effective way to detect fire initiation, therefore, is to detect end products of the oxidation process. With the exception of a few very specialized chemical fires (i.e., fires involving chemicals other than the commonly encountered hydrocarbons), there are three elemental entities (carbon, oxygen, and hydrogen) and three compounds (carbon dioxide ("CO.sub.2 "), carbon monoxide, and water vapor) that are invariably involved in the chemical reactions or combustion of a fire.
Of the three effluent gases generated at the onset of a fire, CO.sub.2 is the best candidate for detection by a fire detector. This is so because water vapor tends to condense easily on every available surface, causing its concentration to fluctuate wildly depending upon the environment and making it difficult to measure. Carbon monoxide is invariably generated in a lesser quantity than CO.sub.2, especially at the beginning of a fire. Although significant amounts of carbon monoxide are produced at fire temperatures of greater than 600.degree. Celsius, these amounts still do not equal the amounts of CO.sub.2 concurrently produced. In addition to being generated abundantly from the start of the fire, CO.sub.2 is a very stable gas.
Although it has been theorized for many years that detection of CO.sub.2 would provide an alternative way to detect fires, CO.sub.2 detectors are not widely used as fire detectors because past CO.sub.2 detectors suffer drawbacks related to cost, moving parts, or false alarms. However, recent advances in the field of Nondispersive Infrared (NDIR) techniques have opened up the possibility of a viable CO.sub.2 detector.
In U.S. Pat. No. 5,053,754 by Jacob Y. Wong entitled "Simple Fire Detector," a fire detector using NDIR techniques is proposed. A beam of 4.26-micron light is directed through a sample of room air to measure the concentration of CO.sub.2 because CO.sub.2 has a strong absorption peak at this wavelength. Both the concentration and the rate of change of concentration of the CO.sub.2 are measured, enabling an alarm to be generated whenever either of these measured values exceeds its respective threshold value. Preferably, an alarm is sounded only if both of these values exceed their respective threshold values. The device is considerably simplified by the use of a window to the sample chamber that is highly permeable to CO.sub.2 but keeps out particles of dust, smoke, oil, and water.
In U.S. Pat. No. 5,079,422 by Jacob Y. Wong entitled "Fire Detection System Using Spatially Cooperative Multi-Sensor Input Technique," individual sensors of a set of N sensors are spaced throughout a large room or unpartitioned building. Comparison of data from different sensors provides information that is unavailable from only a single sensor. The data from each of these sensors and/or the rate of change of such data are used to determine whether a fire has occurred. The use of data from more than one sensor reduces the likelihood of a false alarm.
In U.S. Pat. No. 5,103,096 by Jacob Y. Wong entitled "Rapid Fire Detector," a blackbody source produces a light that is directed through a filter that transmits light in two narrow bands at the 4.26-micron absorption band of CO.sub.2 and at 2.20 microns, at which none of the atmospheric gases has an absorption band. A blackbody source is alternated between two fixed temperatures to produce light directed through ambient gas and through a filter that allows only these two wavelengths of light to pass. To avoid false alarms, an alarm is generated only when both the magnitude of the ratio of the measured intensities of these two wavelengths of light and the rate of change of this ratio are exceeded.
In U.S. Pat. No. 5,369,397 by Jacob Y. Wong entitled "Adaptive Fire Detector," a fire detector that includes a CO.sub.2 sensor and a microcomputer is described that can alter the threshold detection level for CO.sub.2 before an alarm is sounded to compensate for variations in the background concentration of CO.sub.2.
Because virtually all fires generate CO.sub.2, CO.sub.2 detectors should be able to be used as fire detectors. However, two practical limitations have to be dealt with in designing a CO.sub.2 fire detector.
First, although fires generate copious amounts of CO.sub.2, one other commonly encountered type of source--people--also must be taken into account. The concentration level and rate of alarm thresholds for CO.sub.2 fire detectors cannot be set arbitrarily low, because CO.sub.2 generated by people's respiration in an enclosed space might be interpreted as a real fire. In practice, the rate of CO.sub.2 generation by a typical fire can exceed that of human presence by several orders of magnitude. Thus, it is possible to see a CO.sub.2 rate of rise threshold which exceeds the CO.sub.2 rate of rise likely to be caused by human presence and yet is low enough to quickly detect most fires. Some types of smoldering fires, however, generate such small amounts of CO.sub.2 that they are indistinguishable from human presence on the basis of rate or rise of CO.sub.2.
Second, until the cost of an NDIR CO.sub.2 detector is economically attractive, the consumer will be unwilling to purchase this improved fire detector. The concomitant effort to simplify and reduce the cost of an NDIR CO.sub.2 detector is therefore important and relevant in introducing the currently disclosed practical and improved fire detector.
In U.S. Pat. No. 5,026,992, the present inventor began a series of disclosures on the novel simplification of an NDIR gas detector with the ultimate goal of reducing the cost of this device so it can be used affordably to detect CO.sub.2 gas in its application as a fire detector. In U.S. Pat. No. 5,026,992, a spectral ratioing technique for NDIR gas analysis using a differential temperature source was disclosed that leads to an extremely simple NDIR gas detector comprising only one infrared source and one infrared detector.
In U.S. Pat. No. 5,163,332, the present inventor disclosed the use of a diffusion type gas sample chamber in the construction of an NDIR gas detector that eliminated virtually all the delicate and expensive optical and mechanical components of a conventional NDIR gas detector. In U.S. Pat. No. 5,341,214, the present inventor expanded the novel idea of a diffusion type sample chamber of U.S. Pat. No. 5,163,332 to include the conventional spectral ratioing technique in NDIR gas analysis. In U.S. Pat. No. 5,340,986, the present inventor extended the disclosure of a diffusion type gas chamber in U.S. Pat. No. 5,163,332 to a "re-entrant" configuration, further simplifying the construct of an NDIR gas detector.
There have been suggestions to combine different types of fire detectors to achieve economy of production by avoiding duplication of portions of the circuitry, and to provide information about which fire byproduct has been detected. In addition, there has been a suggestion for detecting a pre-fire condition, such as the presence of a hydrocarbon gas, to set a low threshold for the detection of a fire product. Unfortunately neither one of these options addresses the problem of setting a fire product detection threshold that permits the rapid detection of a common fire without resulting in the issuance of an inconveniently and dangerously high level of false alarms. Providing a detector for each of two or more different fire products and separately examining each detector output typically provides a fuller range of sensitivity to various types of fires. For example, an ionization smoke detector in conjunction with a photoelectric smoke detector will detect both smoldering and flaming fires. An alternative option is to combine a detector for a fire product gas with a smoke detector. The great weakness of this approach, however, is that the fire product concentrations could be just below both thresholds, i.e., the fire product gas threshold and the smoke threshold. As a result, this approach still has the potentially fatal shortcoming of failing to detect many types of fires in the beginning stages.